1. Field of the Invention
This invention relates to rectifiers for electric current.
2. Prior Art
Known rectifiers can be made of material such as galena, selenium, copper oxide and germanium. Although rectifiers containing these materials are useful for many purposes, they are not generally satisfactory in high temperature operation. For example, selenium rectifiers are not satisfactory at temperatures over 125.degree. C., copper oxide is not satisfactory at over 85.degree. to 90.degree. C. Galena is unsatisfactory at over 70.degree. C. and germanium is unsatisfactory over 90.degree. C.
Nevertheless, there are applications where high temperature rectifiers are required. For example, in some automotive applications the temperature in and around the engine exhaust is relatively high and there is a need for rectifying electrical circuit elements. Without the ability to endure such high temperature, the rectification components impose serious constraints on the design and must typically be positioned in the passenger compartment. This, of course, undesirably adds to the cost and complexity of the system.
The prior art also teaches titanium dioxide rectifiers which can perform rectification at temperatures in excess of 200.degree. C. However, the structure and method of manufacture disclosed in U.S. Pat. Nos. 2,699,522 and 2,887,633 have an undesirable complexity. The structure taught in both patents utilizes a titanium dioxide ceramic between dissimilar electrode material thus forming an asymmetrical structure with respect to choice of electrode materials. The cause of rectification can be described in terms of Schottky boundary layers between a semiconductor ceramic and a dissimilar electrode. In U.S. Pat. No. 2,887,633 there is taught coating an inorganic artificial barrier material on a titanium dioxide semiconductor. The coating is obtained by vaporizing silicon monoxide in a vacuum and condensing the vapor on the surface of the semiconducting titanium dioxide. An alternative solution both in structure and method would be desirable for achieving a high temperature rectifier.